Weakly electric fish produce Electric Organ Discharges (EODs) which can easily to recorded in a non-invasive manner by monitoring the oscillations of the current field which surrounds an animal. These recordings reflect the activity of a neuronal oscillator, the medullary pacemaker nucleus. Adult individuals of the genus Sternopygus discharge their organ at a sex-specific frequency, males in the range of 50 to 80 Hz, females approximately one octave higher. This sex-specific difference in EOD frequency enables courting individuals to determine the sex of their partners ( ). In preliminary experiments, we hae demostrated that daily injections of testosterone, as well as dihydrotestosterone, lower the EOD frequencies of Sternopygus. This holds true for both male and female adults and juveniles, with the exception that testosterone had no effect upon juveniles. After cessation of injections, the EOD frequencies rebounded within a matter of days towards pre-treatment values. Estradiol 17B elicited increases in discharge frequencies, but only when administered at very high dosages. We plan to study the effects of endocrines upon the EOD with injection therapies, implants, and gonadectomies. Labelled steroid hormones will be injected to identify steroid binding sites in specific parts of the CNS, such as the pacemaker nucleus which drives the electric organ at its specific frequency, and the prepacemaker nucleus in the midbrain, the only neuronal input to the pacemaker ( ). Preliminary studies into the biochemical basis for the effects of endocrines upon this behavior will be undertaken and we hope to measure steroid hormone levels in natural populations, relating sex differences in plasma levels to the results of laboratory studies. The availability of a simple and natural behavioral assay, the reversability of the effects of hormones, and our detailed knowledge about the neurophysiology and neuroanatomy of the brains of these animals make this system an ideal model for the study of hormonal control of neuronal circuits and the ontogeny of sex-specific behaivor.